1. Field of the Invention
The present invention is generally concerned with progressive ophthalmic lenses, that is to say ophthalmic lenses in which at least one surface, the front surface, for example, has along at least part of at least one meridian a curvature which varies progressively so that along at least the relevant part of said meridian the power varies continuously between two powers, one representing far vision and the other near vision.
2. Description of the Prior Art
In practise the power is the combined result of the concave optical surface and the convex optical surface and it is as if there were added to a basic, spherical power representing far vision an additional power representing near vision. For convenience, the lens is manufactured by molding it between two shells between which there is an annular seal, the lens being defined within a circular contour.
At present the progressively varying curvature surface of a progressive ophthalmic lens usually extends to the full aperture.
In other words, it extends to the edge of the lens which in practise has a circular contour, along at least part of this edge.
As a result, the offset of this edge relative to a reference plane perpendicular to the geometrical axis of the lens, in other words the distance between this edge and this plane, varies over at least part of the perimeter.
In practise, the overall thickness of an ophthalmic lens at present varies along this perimeter.
This makes the manufacture of a progressive ophthalmic lens of this kind relatively complicated.
In such manufacture it is important that, to be able to seal effectively the molding cavity that it delimits, the seal employed has a thickness varying along its perimeter, matching the bearing surfaces of the shells.
At this time it is therefore necessary to have one seal for each additional power and, when assembling the seal with the two shells that it separates, to orient it very accurately with respect to the shells.
For esthetic reasons a progressive ophthalmic lens must be as flat as possible and to be comfortable to wear it must be as light as possible so that a positive power lens must be as thin as possible at the center and a negative power lens must be as thin as possible at the periphery.
For one highly specialized type of ophthalmic lens, in practise for high-power ophthalmic lenses for aphakia, it has already been proposed to restrict the surface having a progressively varying curvature to the central part of a progressive ophthalmic lens of this kind and to surround this central area, defining the optically active area of the lens, with a continuous peripheral joining area merging tangentially with the central area. In practise, in lenses manufactured this way the peripheral joining area merges with the optically active central area along a closed curve which, when projected onto a plane perpendicular to the geometrical axis of the lens, has a circular contour, is concave at all points and its radial cross-section, in other words its cross-section in a plane passing through the geometrical axis of the lens, is a circular arc whose radius is constant along its entire perimeter.
As a result, and as previously, a different seal is required for each different basic power and additional power and, again as previously, the seal must be carefully oriented when it is fitted between the two shells it separates.
The number of different seals required is therefore relatively large.
A general object of the invention is an arrangement whereby the number of seals is reduced and use of the seals is simplified, with additional advantages.